Problem: Which of these numbers is prime? ${19,\ 21,\ 42,\ 57,\ 65}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 19 are 1 and 19. The factors of 21 are 1, 3, 7, and 21. The factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42. The factors of 57 are 1, 3, 19, and 57. The factors of 65 are 1, 5, 13, and 65. Thus, 19 is a prime number.